Figure 3-49. Xtl 1500 Tx Signal Path - Motorola ASTRO XTL 1500 Detailed Service Manual

3-74
The multiplexer allows TX modulation audio to be routed from one of 3 possible sources: mobile
microphone (MIC_HI), AUX_MIC (audio sourced from the J2 rear accessory connector in motorcycle
configuration), or AUX_TX (audio from a VRS). The resulting audio source is then sent to a two-
stage, programmable gain/attenuation circuit comprised of U0201 and U0202. The gain is
accomplished via a programmable digital potentiometer in the amplifier circuit. This gain is
adjustable via CPS and is programmed in ±3 dB steps. After passing through an anti-aliasing filter,
audio is sent to the input of the CODEC (U0200) where it is digitized via its internal A/D converter.
The digital audio data is then sent via the SAP SSI bus to the DSP at the Patriot IC. As with speaker
data samples, the DSP reads the microphone audio samples and processes, pre-emphasizes,
filters, and adds signaling information to this data. As with the received trunking data, low-speed
transmit data is processed by the MCU and returned to the DSP. For secure messages, the digital
signal data can be passed to the secure module prior to DSP processing and modulation. The DSP
transfers the data to and from the secure module through the SAP SSI transmit (TXD) and receive
(RXD) lines.
Patriot
U100
SCKA
SRDA
SCKA
512kHz
TX Data
8kHz
After the DSP has finished its processing, filtering, and signaling on the audio data samples, it then
sends this data to the modulation DAC (U0900) on the BBP SSI port, where it is converted to the
analog modulation signal. The data is clocked over to the modulation DAC at a 2.4 MHz bit rate, with
a frame sync (representing the transmit data sample rate) of 48 kHz. Both of these clocks are
generated by the Urchin IC (U0901).
The modulation DAC audio output signal is sent to a switched capacitor low-pass filter (FL0900) that
performs anti-aliasing filtering. The filter output is sent through a multiplexer switch, U0902, and
finally on to the FGU/LV Frac-N synthesizer for modulation of the RF carrier signal.
June 15, 2005
SPI
S PI_SC
SPI_MOSI
SPI_MISO
2.4MHz
SCKB
SAP BBP
MOD Data
STDB
48kHz
SC2B
CODEC
U0200
MCLK
TG
DT
FSR

Figure 3-49. XTL 1500 TX Signal Path

Theory of Operation: Controller Section
URCHIN Clock
IC
2.4MHz
U0901
48kHz
Modulation DAC
SCK
U0900
STD
FM OUT
SFS
Gain/Attenuation
Stages (U0201, U202)
Low Pass Filter
FL0900
Input Output
MODIN
(To LV FRACN)
MIC_HI
(J0401-4)
MAEPF-27890-O
6815854H01-A